1. Field of the Invention
This invention relates to methods and means of altering the frequency of a voltage controlled oscillator (VCO) by summing an external frequency control voltage of either (+) or (-) polarity.
2. Prior Art
U.S. Pat. No. 3,996,531 issued to Wittlinger discloses a voltage controlled circuit (described as an oscillator but more accurately it is a constant width pulse generator with a variable repetition rate) wherein the frequency is determined by the charging and discharging of a capacitor. The capacitor is alternately charged and discharged through separate resistor-diode paths. The rate of discharge of the capacitor is controlled by a control voltage. The control voltage can be increased or decreased by the application of a frequency control voltage to one terminal of an operational amplifier.
The frequency control voltage is increased or decreased to decrease or increase, respectively, the frequency of the output. The voltage drop across the diodes in the resistor-diode charge and discharge paths is a function of temperature. This in turn can affect the voltage applied to the capacitor. To compensate for this diode temperature dependence, two other diodes are employed.
Wittlinger does not compensate for frequency changes due to the temperature dependence of resistors or capacitors themselves. Further, Wittlinger's circuit is more of a variable pulse rate circuit rather than a traditional oscillator. Also, Wittlinger's frequency control voltage range is not centered about ground potential, a feature which is particularly advantageous in servo and feedback amplifier applications where systems are designed to operate about a null.